Patent Document 1 discloses a technique in which a secondary battery is subjected to a pulse discharge with a constant current at a frequency of 100 Hz or greater, a voltage difference of the secondary battery between before and immediately after a pulse discharge is determined, and a dischargable capacity from fully charged state or a degradation degree of the secondary battery from this voltage difference is detected.
Further, Patent Document 2 discloses a technique in which data of a voltage and a current of a secondary battery mounted to an actual vehicle is acquired, and the data is converted to a frequency domain by Fourier transformation to determine an impedance spectrum. Then, on the basis of the determined impedance spectrum, constant fitting in an equivalent circuit model of the secondary battery is performed to determine a resistance component and double layer capacity component of the secondary battery, and then the state of the secondary battery is detected on the basis thereof.